The initial reports of the
Intergovernmental Panel on Climate Change (IPCC 1990) and those
that have followed (IPCC 2001) have consistently noted that our
climate is changing and that one of the most important impacts
of climate change is on the world's water supplies. These
impacts include increased temperatures, changed precipitation,
and shifts in the historic hydrologic cycle. The impact of
climate change on water supplies in the Pacific Northwest is of
particular interest because of the interplay of two factors,
rainfall and temperature. Changes in temperature and
precipitation alter the delicate interaction between the amount
of precipitation that falls as either rain or snow, the eventual
accumulation of snow during the winter, and temporal variability
with which this snow melts and flows through the watershed.
This study explores the impact
that climate change may have on the hydrology of the Bull Run
basin and Portland Water Bureau's (PWB) ability to provide
reliable water to its customers. The study uses a series of
loosely linked models to address potential impacts of climate
change. These models simulate three aspects of process: the
climate, the hydrologic cycle, and water supply system
management.
Currently, the PWB makes use of
two major dams in the Bull Run watershed and groundwater sources
to meet its water demands. The capacity of the dams are small
relative to the flows delivered from their watersheds, thus they
have historically refilled annually. A number of significant
droughts have occurred since 1950 that are used to evaluate
system performance, most commonly the droughts of 1987 and 1992.
In addition, the year 1982 is often used to represent an average
year in system evaluations.
The potential climate change
impacts to be experienced in the Bull Run watershed are
estimated using four different Global Circulation Models. These
models are Department of Energy’s Parallel Climate Model (PCM),
the Max Planck Institute’s ECHAM model and the Hadley Centre’s
HadCM2 and HadCM3 models. These models incorporate a one percent
increase in carbon dioxide per year. They are among the most
highly respected climate models currently available. Monthly
changes in temperature generated by these models suggest a
general warming trend for the decade 2020 (about 1.5° C) and for
the decade 2040 (about 2° C). The warming during the summer
months is slightly greater than during the winter months.
Precipitation also increases, by an average of about 5-8% in
both winter and summer, with the winter increases being larger
than the summer increases. Scientists suggest a high confidence
in this estimate of the temperature change, while there is less
confidence in exact magnitude of the precipitation change.
These changes in temperature and
precipitation have a direct impact on the hydrology of the
basin. The Distributed Hydrology Soil Vegetation Model (DHSVM)
is used to combine historic climate conditions with the climate
change signals from the GCMs to obtain climate-altered
streamflows. The average effect of climate change on the
streamflows is that winter flows are some 15% greater with
climate change (2040) and that late spring flows are some 30%
lower. This change is due to several factors: an increase in
precipitation falling as rain rather than snow in the winter
months, a decrease in the maximum winter snowpack, and an
increase in the rate at which snowpack is melted. Although the
individual GCM results differ somewhat, the general trends are
the same. It is important to note that under current climate
conditions, the Bull Run watershed is a rain-driven system, with
monthly averaged winter flows greater than monthly averaged
spring flows.
The impacts of these
climate-altered streamflows on water supply performance are
evaluated using the Storage and Transmission Model (STM) for
several demand years (2000, 2020, and 2050) and for several
system configurations. Conclusions derived from these
evaluations can be summarized as follows:
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The simulated
impacts of climate change on a drought, such as that which
occurred in 1987 (the system design drought), are not large.
This is because the 1987 drought resulted from a late return
of fall rains. The 2020 and 2040 climate change signals
reinforce, but do not exacerbate that condition.
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In approximately
30% of the years, climate change impacts by the year
2040 would decrease minimum system storage by more than 1
billion gallons each year. This decrease results from earlier
spring runoff that can not be captured in the reservoirs and
lower summer flows due to the earlier streamflow recessions.
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The shift in the
timing and volume of spring runoff in the Bull Run basin
associated with climate change, particularly by 2040, will
decrease the average maximum winter snowpack. This will result
in an increase in the frequency of low flow in early summer.
This shift will result in a number of droughts as extreme as
1992. |
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Continued growth
in the M&I demand will have a more crucial impact on minimum
annual reservoir storage than climate change. Growing water
demands will likely decrease the average minimum reservoir
storage by 4 billion gallons by 2050. |
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The current
system configuration provides sufficient storage to meet the
year 2000 demands given the current hydrology and the
climate-altered streamflows of 2020 and 2040. |
